The invention relates generally to the field of isolated power supplies, and more particularly to an arrangement providing EMI suppression via a shielded common mode choke coupled to the secondary ground.
Electro-magnetic interference (EMI) also called radio frequency interference (RFI) is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. Broadband interference usually comes from incidental radio frequency emitters, including, but not limited to, electric power transmission lines, electric motors, thermostats, computers and other digital equipment as well as televisions. Any circuit wherein electrical power is turned off and on rapidly is a potential source of EMI. The rich harmonic content of these various devices means that they can produce EMI having a very broad spectrum.
One characteristic of broadband EMI is an inability to filter it effectively once it has entered the receiver chain, and as a result received broadband EMI may interrupt, obstruct, or otherwise degrade or limit the effective performance of the receiving circuit. Most countries have requirements that mandate electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and the electronic and electrical hardware should not emit EMI of a strength which could interfere with other equipment.
One common method used for suppressing EMI generation by switching power supplies is to utilize common mode chokes as illustrated in FIG. 1 as a power supply arrangement 10. Power supply arrangement 10 comprises: a primary side 20 of an isolated power supply 25; a secondary side 30 of isolated power supply 25; a load 40; and an EMI suppression circuit 45. EMI suppression circuit 45 comprises: a resistor 50; a first and a second capacitor 60; and a first and a second common mode choke 70. A first connection of an AC mains, denoted AC-LINE, is connected to a first end of resistor 50, to a first end of first capacitor 60 and to a first end of a first winding 72 of first common mode choke 70. A second end of first winding 72 of first common mode choke 70 is connected to a first end of second capacitor 60 and to a first end of a first winding 72 of second common mode choke 70. A second end of first winding 72 of second common mode choke 70 is connected to a first input of primary side 20 of isolated power supply 25.
A second connection of the AC mains, denoted AC-NEUTRAL, is connected to a second end of resistor 50, to a second end of first capacitor 60 and to a first end of a second winding 74 of first common mode choke 70. A second end of second winding 74 of first common mode choke 70 is connected to a second end of second capacitor 60 and to a first end of a second winding 74 of second common mode choke 70. A second end of second winding 74 of second common mode choke 70 is connected to a second input of primary side 20 of isolated power supply 25.
Primary side 20 of isolated power supply 25 is coupled to secondary side 30 of isolated power supply 25 in an isolated manner, illustrated without limitation as comprising a transformer. In one particular embodiment, isolated power supply 25 is arranged as a flyback transformer, however this is not meant to be limiting in any way. The output of secondary side 30 is connected to load 40, with the return of each of load 40 and secondary side 30 connected to a common secondary side potential, such as a chassis or ground.
First and second capacitors 60 are thus connected so as to see the input AC line voltage, and as EMI suppression capacitors are thus denoted X capacitors in accordance with EN 60384-14 and IEC 60384-14. Two stages are illustrated, however this is not meant to be limiting in any way, and one stage of EMI suppression may be implemented when EMI is not heavy.
Common mode chokes 70 utilize their inductance to attenuate the switching power noise emanating from isolated power supply 25 from being emitted through the input power line. Unfortunately, common mode chokes 70 exhibit parasitic capacitance between various turns of its constituent wires, i.e. between turns of first winding 72 and between turns of second winding 74 as shown in FIG. 2, which further illustrates a common core 76. The parasitic capacitance allows RF noise to bypass inductance of common mode choke 70. Furthermore, common mode choke 70 acts as an antenna for radiative electromagnetic noise of high frequency. Supplying a shield for common mode choke 70 is ineffective, as the shield itself acts as an antenna for emitting coupled noise. The use of high voltage capacitors between AC mains lines to the secondary common point may be helpful, but high voltage capacitors add significantly to cost.
There is thus a long felt need for an arrangement which provides a reduction in EMI without increasing cost dramatically.